Method for forming fluorinated ionomers

ABSTRACT

This invention concerns a method for forming ionomers by treatment with ammonium carbonate of copolymers having a substantially fluorinated, but not perfluorinated, polyethylene backbone having pendant groups of fluoroalkoxy sulfonyl fluoride. Ionomers derived therefrom by ion exchange are useful in electrochemical applications such as batteries, fuel cells, electrolysis cells, ion exchange membranes, sensors, electrochemical capacitors, and modified electrodes.

FIELD OF THE INVENTION

This invention concerns ammonium ionomers, and a method for preparingthem by contacting a polymer having a substantially fluorinated, but notperfluorinated, polyethylene backbone having pendant groups offluoroalkoxy sulfonyl fluoride, with an excess of ammonium carbonatesolution. This invention further concerns a method for forming ionomersby ion exchange with ammonium ionomers. The ionomers so formed areuseful in electrochemical applications such as batteries, fuel cells,electrolysis cells, ion exchange membranes, sensors, electrochemicalcapacitors, and modified electrodes.

BACKGROUND OF THE INVENTION

Copolymers of vinylidene fluoride (VDF) with vinyl alkoxy sulfonylhalides are known in the art.

The disclosures in Ezzell et al. (U.S. Pat. No. 4,940,525) encompasscopolymers of VDF with vinyl ethoxy sulfonyl fluorides containing oneether linkage. Disclosed is a process for emulsion polymerization oftetrafluoroethylene (TFE) with the vinyl ethoxy comonomer.

Banerjee et al. (U.S. Pat. No. 5,672,438) disclose copolymers of VDFwith vinyl alkoxy sulfonyl fluorides containing more than one etherlinkage.

Connolly et al. (U.S. Pat. No. 3,282,875) disclose the terpolymer of VDFwith perfluorosulfonyl fluoride ethoxy propyl vinyl ether (PSEPVE) andhexafluoropropylene (HFP). They broadly teach an emulsion polymerizationprocess said to be applicable to copolymerization of vinyl ethers withany ethylenically unsaturated comonomer, with greatest applicability tofluorinated monomers.

Barnes et al. (U.S. Pat. No. 5,595,676) disclose “substantiallyfluorinated” copolymers of a vinyl ether cation exchangegroup-containing monomer with a “substantially fluorinated” alkene. Thecopolymer is produced by controlled addition of the alkene in emulsionpolymerization, followed by hydrolysis in NaOH. PSEPVE/TFE copolymersare exemplified.

Hietala et al., J. Mater. Chem. Volume 7 pages 721–726, 1997, disclose aporous poly(vinylidene fluoride) on to which styrene is grafted byexposing the PVDF to irradiation. The styrene functionality issubsequently functionalized to sulfonic acid by exposure of the polymerto chlorosulfonic acid. The resultant acid polymer, in combination withwater, provides a proton-conducting membrane.

Formation of ionomers and acid copolymers by hydrolysis of the sulfonylfluoride functionality in copolymers of TFE and fluoro alkoxy sulfonylfluorides is well known in the art. The art teaches exposure of thecopolymer to strongly basic conditions.

See for example, Ezzell et al. U.S. Pat. No. 4,940,525, wherein is used25 wt % NaOH(aq) for 16 hours at 80–90° C.; Banerjee et al. U.S. Pat.No. 5,672,438, wherein is used 25 wt % NaOH for 16 hours at 90° C., or,in the alternative, an aqueous solution of 6–20% alkali metal hydroxideand 5–40% polar organic liquid (e.g., DMSO) for 5 minutes at 50–100° C.;Ezzell et al. U.S. Pat. No. 4,358,545 wherein is used 0.05N NaOH for 30minutes for 50° C.; Ezzell et al. U.S. Pat. No. 4,330,654, wherein isused 95% boiling ethanol for 30 minutes followed by addition of equalvolume of 30% NaOH (aq) with heating continued for 1 hour; Marshall etal. EP 0345964 A1, wherein is used 32 wt % NaOH (aq) and methanol for 16hours at 70° C., or, in the alternative, an aqueous solution of 11 wt %KOH and 30 wt % DMSO for 1 hour at 90° C.; and, Barnes et al. U.S. Pat.No. 5,595,676, wherein is used 20 wt % NaOH (aq) for 17 hours at 90° C.

Because of its high dielectric constant, high electrochemical stability,and desirable swelling properties, poly(vinylidene fluoride) is known inthe art of lithium batteries as a highly desirable material for use as amembrane separator. For example Gozdz et al. (U.S. Pat. No. 5,418,091)disclose porous PVDF homopolymer and copolymer containing solutions oflithium carbonates in aprotic solvents useful as separators in lithiumbatteries.

Porous membranes of the type described by Gozdz, however, conduct boththe cation and the anion back and forth across the separator, and arethus subject to concentration polarization during use, which degradesthe performance of the battery in which it is used. So-called single ionconducting polymeric membranes, wherein the ionic carbonate is attachedto the polymer backbone, thereby immobilizing either the cation or theanion, offer a solution to the concentration polarization problem, andare known in the art. One particularly well-known such single ionconducting polymer is Nafion® Perfluoroionomer Resin and Membranesavailable from DuPont, Wilmington, Del. Nafion is a copolymer of TFE andperfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) which has beenhydrolyzed by treatment with an alkali metal hydroxide according to theteachings of the art as hereinabove described.

It is further known in the art, and hereinbelow shown, that PVDFhomopolymers and copolymers are subject to attack by strong bases suchas the alkali metal hydroxides taught in the art hereinabove cited. Ofparticular importance is that the attack of basic nucleophiles on acopolymer of VDF and perfluorovinyl ethers results in the removal of thevinyl ether moiety from the polymer, see W. W. Schmiegel in DieAngewandte Makromolekulare Chemie, 76/77 pp 39ff, 1979. Since the highlypreferred monomer species taught in the art, and exemplified by DuPont'sNafion and similar products, for imparting ionomeric character tovarious polymers is a vinyl ether terminated by a sulfonyl halidefunctionality, the sensitivity to base attack of the VDF copolymerformed therewith has prevented the development of a single-ionconducting ionomer based upon VDF. There simply is no means taught inthe art for making the ionomer.

Doyle et al, U.S. Pat. No. 6,025,092, discloses ionomers formed withvinylidene fluoride copolymers by subjecting sulfonyl fluoridecontaining precursors to hydrolysis with alkali and alkaline earth metalcarbonates, such as lithium carbonate, under mildly basic conditions.The method of Doyle et al, however is limited in that any excess overstoichiometric amounts of the hydrolyzing agent results in attack on theVDF backbone, causing polymer degradation. Thus the method of Doyle etal is limited in industrial applicability.

Barton et al, WO 0052085A1, discloses melt processible compositionscomprising alkali metal ionomers having vinylidene fluoride monomerunits, and liquids imbibed therewithin. The ionomers of Barton et al arenot melt processible without incorporation of the liquids.

SUMMARY OF THE INVENTION

The present invention provides for an ionomer comprising monomer unitsof vinylidene fluoride and 0.5–50 mole % of a perfluoroalkenyl monomerhaving a pendant group of the formula—(O—CF₂CFR)_(a)O—CF₂(CFR′)_(b)SO₃ ⁻NH₄ ⁺wherein R and R′ are independently selected from F, Cl or aperfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, b=0to 6.

The present invention also provides for a process for forming an ionomercomprising

-   -   contacting a polymer comprising        -   monomer units of vinylidene fluoride and 0.5–50 mole % of a            perfluoroalkenyl monomer having a pendant group of the            formula            —O—CF₂CFR)_(a)O—CF₂(CFR′)_(b)SO₂F        -    wherein R and R′ are independently selected from F, Cl or a            perfluorinated alkyl group having 1 to 10 carbon atoms, a=0,            1 or 2, b=0 to 6,    -    with an excess of a solution of ammonium carbonate for a period        of time sufficient to obtain the degree of conversion desired to        the ammonium sulfonate form of the polymer.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of description in the present invention, the genericterm “ionomer” will be taken to encompass the ammonium sulfonate and thesulfonic acid forms of the polymer of the invention, as well as thealkali and alkaline earth salts thereof. For the purpose of the presentinvention, the term “excess” when applied to the ammonium carbonatesolution of the present invention means that the solution contains more,preferably many fold, more than the amount of ammonium carbonatenecessary to achieve complete hydrolysis of the sulfonyl fluoride to thesulfonate based upon reaction stoichiometry. That is, “excess” meansbeyond, preferably many fold beyond, the stoichiometric amount.

The term “substantially fluorinated” means that at least 50 mole % ofthe hydrogens of the corresponding polyethylene backbone have beenreplaced by fluorines.

In one aspect of the present invention the sulfonyl fluoride-containingprecursor polymer is contacted with a many fold excess of ammoniumcarbonate solution, effecting the hydrolysis of the sulfonyl fluoride tothe ammonium sulfonate without degradation of the polymer backbone. Inanother aspect of the present invention, the ammonium sulfonate ionomermay be melt processed, such as by thermal consolidation of ammoniumsulfonate ionomer of the invention into a shaped article such as apolymer film, without the addition of any liquid to the polymer.

Means for forming the ammonium sulfonate ionomer into a film, sheet orother shaped article include melt pressing and extrusion using a screwextruder. Other means include roll milling and such other meanswell-known in the art of plastics processing for forming shaped articlesof thermoplastic polymers. The ammonium sulfonate ionomer of theinvention can also be formed into shaped articles according to solutionmethods disclosed in the art such as by dissolution in a solventfollowed by solution casting of a film or sheet upon a substrate.However melt processing is preferred.

In an alternative embodiment of the present invention, the sulfonylfluoride form of the polymer is first melt-formed into a sheet and thencontacted with an excess of ammonium carbonate solution to effecthydrolysis to the ammonium sulfonate ionomer.

In a further embodiment, the ammonium sulfonate ionomer is contactedwith a mineral acid, preferably an aqueous mineral acid, such as nitricacid, to form the sulfonic acid ionomer which is useful in fuel cells.In yet a further embodiment of the invention, the sulfonic acid ionomeris contacted with a solution, preferably an aqueous solution, of analkali metal salt, such as LiCl, to form the alkali sulfonate ionomeruseful in various electrochemical cells such as lithium batteries.

In a further embodiment, the ammonium ionomer may be contacted with asolution, preferably an aqueous solution, of an alkali metal salt suchas LiCl to form the alkali metal ionomer by ion exchange. It ispreferred, however, to first form the sulfonic acid followed by ionexchange to form the alkali metal, preferably the lithium, ionomer.

In all said foregoing embodiments, it is preferred that the ionomerundergoing the ion exchange processes be in the form of a film or sheet.

In the process of the invention vinylidene fluoride (VDF) iscopolymerized with a non-ionic monomer (I) represented by the formulaCF₂═CF—(O—CF₂CFR)_(a)O—CF₂(CFR′)_(b)SO₂F  (I)where R and R′ are independently selected from F, Cl or a fluorinated,preferably perfluorinated, alkyl group having 1 to 10 carbon atoms, a=0,1 or 2, b=0 to 6. Preferably R is trifluoromethyl, R′ is F, a=1 and b=1.In the process of the invention, the copolymer so formed is contactedwith an excess of a solution of ammonium carbonate to form an ionomercomprising monomer units of VDF and 0.5–50 mole %, preferably 0.5–36mole %, of an ionic perfluoroalkenyl monomer having a pendant group ofthe formula—(O—CF₂CFR)_(a)O—CF₂(CFR′)_(b)SO₃ ⁻NH₄ ⁺where R and R′ are independently selected from F, Cl or a perfluorinatedalkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, b=0 to 6.Preferably, R is trifluoromethyl, R′ is F, a=0 or 1, b=1.

The ammonium carbonate solution suitable for use in the presentinvention is a solution formed by adding ammonium carbonate to water,alcohol, organic carbonate, or mixtures thereof. Suitable alcoholsinclude but are not limited to methanol, ethanol and butanol. Suitablecarbonates include but are not limited to ethylene carbonate andpropylene carbonate. Preferably the ammonium carbonate is dissolved in amixture of methanol and water.

A preferred hydrolysis process of the invention comprises contacting thesulfonyl fluoride-containing polymer with an excess of a solution ofammonium carbonate in methanol (optionally containing another solventsuch as water), in the range of ca. 0–85° C., preferably roomtemperature to 65° C. for a sufficient length of time to convert thedesired percentage of sulfonyl fluoride to ammonium sulfonate.

Generally preferred are the mildest hydrolysis conditions possibleconsistent with timely conversion of the sulfonyl fluoride. The severehydrolysis conditions taught in the art for hydrolyzing sulfonylfluoride to sulfonate in the case of ionomers which do not include VDF,cause degradation of the VDF-containing copolymer in the presentinvention. The degree of conversion can be conveniently monitored by thedisappearance of the characteristic infrared absorption band for thesulfonyl fluoride group at about 1462 cm⁻¹. Alternatively, ¹⁹F NMRspectroscopy may be used as described in the examples.

The ionomers prepared by the process of the invention include copolymercompositions in which the ionic monomer unit is present in the ionomerof the invention at concentrations ranging from 0.5 to 50 mole %,preferably 0.5–36 mole %.

Other cationic forms of the ion-exchange membrane can be achieved usingion-exchange procedures commonly known in the art and as outlined hereinabove (see for example Ion Exchange by F. Helfferich, McGraw Hill, NewYork 1962). For example, the protonic form of the membrane is preferablyobtained by immersing the ammonium-ionomer into an aqueous acid.

Silver and copper sulfonate ionomers can be made by ion exchange withthe ammonium sulfonate form of the polymer. For example, repeatedtreatment of the ammonium sulfonate ionomer with an aqueous solution ofa silver salt such as silver fluoride or silver perchlorate wouldproduce at least a partially cation exchanged silver sulfonate ionomer.In a similar fashion, the cuprous sulfonate ionomer can be produced byrepeated treatment of the ammonium sulfonate ionomer with an aqueousacidic solution of a copper salt such as cuprous chloride.

In many applications, the ionomer is preferably formed into a film orsheet. Films of the ionomer may be formed according to processes knownin the art. In one embodiment, the thermoplastic sulfonyl fluorideprecursor is extrusion melt cast onto a cooled surface such as arotating drum or roll, whence it is subject to hydrolysis according tothe process of the invention. In a second embodiment, a sulfonylfluoride-containing polymer is dissolved in a solvent, the solution castonto a smooth surface such as a glass plate using a doctor knife orother device known in the art to assist in depositing films on asubstrate, and the resultant film subject to hydrolysis according to theprocess of the invention. In a third embodiment, the sulfonyl fluoridecopolymer resin is subject to hydrolysis by dissolution or suspension ina hydrolyzing medium, followed by optional addition of cosolvent, andfiltration or centifugation of the resulting mixture, and finallysolvent casting of the ionomer solution onto a substrate using a doctorknife or other device known in the art to assist in depositing films ona substrate.

In an alternative embodiment, it is found in the practice of the presentinvention that the ammonium ionomer is particularly amenable to meltforming. Thus, the ammonium ionomer may be isolated in the form of apowder, and the powder melt formed into a film or sheet which may thenbe subject to ion exchange according to the methods taught herein.

The ionomers prepared according to the practice of the invention may beterpolymers. Suitable third monomers include tetrafluoroethylene,chlorotrifluoroethylene, ethylene, hexafluoropropylene,trifluoroethylene, vinyl fluoride, vinyl chloride, vinylidene chloride,perfluoroalkylvinyl ethers of the formula CF₂=CFOR_(f) where R_(f)=CF₃,C₂F₅ or C₃F₆. Preferred termonomers include tetrafluoroethylene,hexafluoropropylene, ethylene and the perfluoroalkylvinyl ethers.Termonomers are preferably present in the polymer at a concentration ofup to 30 mole %.

EXAMPLES

All chemicals were used as received unless stated otherwise.

The following terms and abbreviations are used in the examples. Theabbreviation “VF2” refers to the monomer 1,1-difluoroethene. Theabbreviation “PSEPVE” refers to2-[1-[difluoro[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2-tetrafluoroethanesulfonylfluoride. The term “MeOH” refers to methyl alcohol.

Differential scanning calorimetry (DSC) was performed according to ASTMD4591, in a nitrogen atmosphere and at a heating rate of 20° C./minute,using a TA Instruments Model 2910.

¹⁹F NMR spectra were recorded using a Bruker AVANCE DRX 400spectrometer.

The below examples were performed on polymer powder which can be pressedinto good films. Films can also be hydrolyzed directly by immersing themin an ammonium carbonate bath (MeOH/H₂O) using this method.

EXAMPLE 1 Hydrolysis of PSEPVE/VF2 Copolymer Using Excess AmmoniumCarbonate in MeOH/H2O

A 4 liter horizontal, stainless-steel stirred polymerization reactorequipped with a 4-bladed agitator was flushed with nitrogen and chargedwith 1.65 liter of demineralized water and 6 g of ammoniumperfluorooctanoate. The reactor was sealed, pressurized with nitrogen to100 psig and then vented to 0 psig. This pressure/venting cycle wasrepeated twice. The reactor was then evacuated to −14 psig then purgedwith vinylidene difluoride (VF2) to 0 psig. This evacuation/purge cyclewas repeated two times. At 0 psig of VF2 in the reactor, 20 ml of anaqueous PSEPVE emulsion (containing 10 g PSEPVE) was injected into thereactor. The reactor contents were agitated at 200 rpm and heated to 60°C. The reactor was pressurized with VF2 to 300 psig at which time 0.9 gpotassium persulfate dissolved in 20 ml demineralized water was injectedat 10 ml/min. The polymerization initiated in 0.07 hr.

A mixture of VF2 and PSEPVE (as PSEPVE emulsion containing 0.5 gPSPEVE/ml) in a 1:1 mole ratio was fed to the reactor at about the rateat which it was consumed maintaining about 300 psig pressure in thereactor. The reaction was continued in this manner until about 215 g ofPSEPVE were fed to the reactor. The feeding of PSEPVE was thendiscontinued and VF2 was fed to the reactor at about the rate at whichit was consumed maintaining about 300 psig pressure in the reactor,until an overall total of 334 g VF2 had been fed to the reactor.

The reactor contents were cooled to ambient temperature, vented to 0psig and discharged as a milky-white polymer emulsion containing 22 wt %polymer. The polymer emulsion was frozen to cause agglomeration of thepolymer particles and their separation from the aqueous phase. Thepolymer agglomerates were filtered and washed vigorously 4 times withfiltered tap water at about 50° C. then with demineralized water atambient temperature. The washed polymer was dried at 100° C. underpartial vacuum with a sweep of nitrogen to yield 520 g of white granularpolymer. DSC analysis showed a glass transition temperature at −24° C.(at the intlection point) and a well defined crystalline melting pointat 166° C. (ΔH_(f)=22.2 J/g) on the second heat. Elemental analysisfound: C, 30.41 wt % from which an average composition of 8.1 mole %PSEPVE and 91.9 mole % VF2 could be calculated. The polymer could bepressed into translucent white slabs and films at 200° C. that wereclean and dense, free of voids or visible color.

A 3-necked 300 ml jacketed flask was equipped with a magnetic stir bar,condenser and nitrogen inlet. The flask was charged with 10 g (˜10 mmole—SO₂F) of the PSEPVE/VF2 copolymer prepared above, 3.85 g (40 mmole) ofammonium carbonate and 100 ml of MeOH/H₂O (50 v %). With gentle stiring,the reaction mixture was heated to 50° C. for 24 hr, then cooled to roomtemperature. The polymer was collected by filtration then washed 4 timesby suspending in distilled water at 25° C. and filtering. The collectedpolymer was dried at 25° C. under vacuum to a white powder. By ¹⁹Fnmranalysis (DMF-d7), 100% of the —SO₂F was converted and the polymercontained 8.2 mole % PSEPVE. The polymer could be pressed at 200° C.into a thin film (5–10 mil) which was colorless, clear and tough.

EXAMPLE 2 Hydrolysis of PSEPVE/VF2 copolymer Film Using Excess AmmoniumCarbonate in MeOH/H₂O and Conversion to the Sulfonic Acid Form

A film, 0.005 in. to 0.007 in. thick, was prepared from the PSEPVE/VF2copolymer powder prepared in Example 1 by melt pressing at 20° C. Thefilm was supported between Teflon™ mesh to prevent it from folding onitself and immersed for 24 hr in a 1-liter stirred bath containing a 0.4molar ammonium carbonate solution in 1:1 methanol/water at 50° C. At theend of this period, the supported film assembly was removed from theammonium carbonate solution, rinsed with several portions ofdemineralized water, then immersed for 18 hr in a second 1-liter stirredbath containing 3 molar nitric acid at 70° C. At the end of this period,the supported film assembly was removed from the nitric acid solution,rinsed with several portions of demineralized water, then immersed inboiling demineralized water for 2 hr. The water was changed severaltimes during the boiling to remove residual nitric acid. At the end ofthis period, the supported film assembly was removed from the boilingdemineralized water and immediately immersed in fresh demineralizedwater at ambient temperature. A 1.0 by 1.5 cm² section of the film wasblotted dry and assembled into a conductivity cell. Proton ionconductivity was measured at ambient conditions according to the methodof Sone et al, J. Electrochem. Soc 143, 1254 (1996), and determined tobe equal to 81×10⁻³ S/cm.

EXAMPLE 3 Hydrolysis of PSEPVE/VF2 Copolymer Using Excess AmmoniumCarbonate in MeOH/H2O

Example 1 was repeated, but heating was at 60° C. for 24 hr for thisexample. By ¹⁹Fnmr analysis (DMF-d7), 100% of the —SO₂F was convertedand the polymer contained 7.6 mole % PSEPVE.

EXAMPLE 4 Hydrolysis of PSEPVE/VF2 Copolymer Using StoichiometricAmmonium Carbonate in MeOH

A 4 liter horizontal, stainless-steel stirred polymerization reactorequipped with a 4-bladed agitator, was flushed with nitrogen and chargedwith 1.65 liter of demineralized water and 6 g of ammoniumperfluorooctanoate. The reactor was sealed, pressurized with nitrogen to100 psig and then vented to 0 psig. This pressure/venting cycle wasrepeated twice. The reactor was evacuated to −14 psig then purged withvinylidene difluoride (VF2) to 0 psig. This evacuation/purge cycle wasrepeated twice. At 0 psig of VF2 in the reactor, 20 ml of an aqueousPSEPVE emulsion (containing 10 g PSEPVE) was injected into the reactor.The reactor contents were agitated at 200 rpm and heated to 60° C. Thereactor was pressurized with VF2 to 300 psig at which time 0.9 gpotassium persulfate dissolved in 20 ml demineralized water was injectedat 10 ml/min. The polymerization initiated in 0.05 hr. A mixture of VF2and PSEPVE (as PSEPVE emulsion containing 0.5 g PSPEVE/ml) in a 4:1 moleratio was fed to the reactor at about the rate at which it was consumedmaintaining about 300 psig pressure in the reactor. The reaction wascontinued in this manner until about 215 g of PSEPVE were fed to thereactor. The feeding of PSEPVE was then discontinued and VF2 was fed tothe reactor at about the rate at which it was consumed maintaining about300 psig pressure in the reactor, until an overall total of 334 g VF2had been fed to the reactor. The reactor contents were cooled to ambienttemperature, vented to 0 psig and discharged as a milky-white polymeremulsion containing 22 wt % polymer. The polymer emulsion was frozen tocause agglomeration of the polymer particles and their separation fromthe aqueous phase. The polymer agglomerates were filtered and washedvigorously 4 times with filtered tap water at about 50° C. then withdemineralized water at ambient temperature. The washed polymeragglomerates were dried at 100° C. under partial vacuum with a sweep ofnitrogen to yield 551 g of white granular polymer. DSC analysis showed aglass transition temperature at −26° C. and a well defined crystallinemelting point at 160° C. (ΔH_(f)=15.9 J/g) on the second heat. Elementalanalysis found: C, 30.28 wt % from which an average composition of 8.3mole % PSEPVE and 91.7 mole % VF2 could be calculated. The polymer couldbe pressed into translucent white slabs and films at 200° C. that wereclean and dense, free of voids or visible color.

A 3-necked 3000 ml flask was equipped with an overhead stirrer, heatingmantle, dropping funnel, distillation head and nitrogen inlet. The flaskwas charged with 50 g (44 mmole —SO₂F) PSEPVE/VF2 copolymer preparedabove, 4.2 g (44 mmole) of ammonium carbonate and 750 ml of MeOH. Thereaction mixture was stirred at 25° C. for 24 hr. The reaction mixturewas heated to reflux to distill off the MeOH. Toluene was added throughthe dropping funnel to maintain the fluid level in the flask. Thedistillation was continued until the distillate contained <1% MeOH byGLC analysis at which time heating was discontinued and the reactionmixture was cooled to 25° C. with stirring. The polymer was collected byfiltration and washed on the filter with 500 ml toluene. The collectedpolymer was dried at 25° C. under vacuum to a white powder. By ¹⁹Fnmranalysis (DMF-d7), 100% of the —SO₂F was converted and the polymercontained 9.5 mole % PSEPVE.

COMPARATIVE EXAMPLES

Comparative examples show other alkali metal carbonates or tetraalkylammonium carbonates are either ineffective at —SO₂F hydrolysis ordegrade the polymer by discoloration and loss of PSEPVE.

Comparative Example A Hydrolysis of PSEPVE/VF2 Copolymer Using ExcessLithium Carbonate in MeOH/H2O

This example demonstrates that lithium carbonate is ineffective atcomplete hydrolysis of the sulfonyl fluoride moiety under the conditionsof Example 1.

A 3-necked 300 ml jacketed flask was equipped with a magnetic stir bar,condenser and nitrogen inlet. The flask was charged with 5 g (˜5 nmole—SO₂F) of the PSEPVE/VF2 copolymer powder prepared in Example 1, 1.5 g(20 mmole) lithium carbonate and 50 ml MeOH/H₂O (50 v %). With gentlestirring, the reaction mixture was heated to 50° C. for 24 hr, thencooled to room temperature. The polymer was collected by filtration thenwashed 4 times by suspending in distilled water at 25° C. and filtering.The collected polymer was dried at 25° C. under vacuum to a whitepowder. By ¹⁹Fnmr analysis (DMF-d7), only 17% of the —SO₂F was convertedand the polymer contained 8.2 mole % PSEPVE.

Comparative Example B Hydrolysis of PSEPVE/VF2 Copolymer Using ExcessLithium Carbonate in MeOH/H2O

This example demonstrates that lithium carbonate is ineffective atcomplete hydrolysis of the sulfonyl fluoride moiety under the conditionsof Example 3.

Comparative Example A was repeated, but heated 60° C./24 hr for thisexample. By ¹⁹Fnmr analysis (DMF-d7), 28% of the —SO₂F was converted andthe polymer contained 7.9 mole % PSEPVE.

Comparative Example C

Hydrolysis of PSEPVE/VF2 Copolymer Using Excess Sodium Carbonate inMeOH/H2O

This example demonstrates that sodium carbonate hydrolysis of thesulfonyl fluoride moiety under the conditions of Example 1 results indegradation of the starting polymer as evidenced by the substantial lossof functional comonomer.

A 3-necked 300 ml jacketed flask was equipped with a magnetic stir bar,condenser and nitrogen inlet. The flask was charged with 10 g (˜10 mmole—SO₂F) of the PSEPVE/VF2 copolymer powder prepared in Example 1, 4.25 g(40 mmole) sodium carbonate and 100 ml MeOH/H₂O (50 v %). With gentlestirring, the reaction mixture was heated to 50° C. for 24 hr, thencooled to room temperature. The polymer was collected by filtration thenwashed 4 times by suspending in distilled water at 25° C. and filtering.The collected polymer was dried at 25° C. under vacuum to a yellowpowder. By ¹⁹Fnmr analysis (DMF-d7), 100% of the —SO₂F was converted andthe polymer contained only 5.9 mole % PSEPVE. The polymer could bepressed at 200° C. into a thin film (5–10 mil) which was colored.

Comparative Example D

Hydrolysis of PSEPVE/VF2 Copolymer Using Excess Sodium Carbonate inMeOH/H2O

This example demonstrates that sodium carbonate hydrolysis of thesulfonyl fluoride moiety under the conditions of Example 3 results indegradation of the starting polymer as evidenced by the substantial lossof functional comonomer.

Comparative Example C was repeated, but heated 60° C./24 hr for thisexample. By ¹⁹Fnmr analysis (DMF-d7), 100% of the —SO₂F was convertedand the polymer contained only 5.1 mole % PSEPVE.

Comparative Example E Hydrolysis of PSEPVE/VF2 Copolymer Using ExcessPotassium Carbonate in MeOH/H2O

This example demonstrates that potassium carbonate is ineffective atcomplete hydrolysis of the sulfonyl fluoride moiety under the conditionsof Example 1. And further, this example demonstrates that potassiumcarbonate hydrolysis of the sulfonyl fluoride moiety under theconditions of Example 1 results in degradation of the starting polymeras evidenced by the substantial loss of functional comonomer.

A 3-necked 300 ml jacketed flask was equipped with a magnetic stir bar,condenser and nitrogen inlet. The flask was charged with 25 g (˜25 mmole—SO₂F) of the PSEPVE/VF2 copolymer powder prepared in Example 1, 13.8 g(100 mmole) potassium carbonate and 250 ml MeOH/H₂O (50 v %). Withgentle stirring, the reaction mixture was heated to 50° C./22 hr, thencooled to room temperature. The polymer was collected by filtration thenwashed 4 times by suspending in distilled water at 25° C. and filtering.The collected polymer was dried at 25° C. under vacuum to a yellowpowder. By ¹⁹Fnmr analysis (DMF-d7), 87% of the —SO₂F was converted andthe polymer contained only 6.1 mole % PSEPVE. The polymer could bepressed at 200° C. into a thin film (5–10 mil) which was colored.

Comparative Example F Hydrolysis of PSEPVE/VF2 Copolymer Using ExcessPotassium Carbonate in MeOH/H2O

This example demonstrates that potassium carbonate hydrolysis of thesulfonyl fluoride moiety under the conditions of Example 3 results indegradation of the starting polymer as evidenced by the substantial lossof functional comonomer.

Comparative Example E was repeated, but heated 60° C./18 hr for thisexample. By ¹⁹Fnmr analysis (DMF-d7), 100% of the —SO₂F was convertedand the polymer contained only 5.6 mole % PSEPVE.

Comparative Example G Hydrolysis of PSEPVE/VF2 Copolymer Using ExcessTetramethylammonium Carbonate in MeOH/H₂O

This example demonstrates that tetramethylammonium carbonate isineffective at complete hydrolysis of the sulfonyl fluoride moiety underthe conditions of Example 1. And further, this example demonstrates thattetramethylammonium carbonate hydrolysis of the sulfonyl fluoride moietyunder the conditions of Example 1 results in degradation of the startingpolymer as evidenced by the substantial loss of functional comonomer.

A 4-L horizontal, stainless-steel stirred polymerization reactorequipped with 4-bladed agitator, was flushed with nitrogen and chargedwith 1.65 liter of demineralized water and 6 g of ammoniumperfluorooctanoate. The reactor was sealed, pressurized with nitrogen to100 psig and then vented to 0 psig. This pressure/venting cycle wasrepeated twice. The reactor was evacuated to −14 psig then purged withvinylidene difluoride (VF2) to 0 psig. This evacuation/purge cycle wasrepeated twice. At 0 psig of VF2 in the reactor, 20 ml of an aqueousPSEPVE emulsion (containing 10 g PSEPVE) was injected into the reactor.The reactor contents were agitated at 200 rpm and heated to 60° C. Thereactor was pressurized with VF2 to 300 psig at which time 0.9 gpotassium persulfate dissolved in 20 ml demineralized water was injectedat 10 m/min. The polymerization initiated in 0.06 hr. A mixture of VF2and PSEPVE (as PSEPVE emulsion containing 0.5 g PSPEVE/ml) in a 2:1 moleratio was fed to the reactor at about the rate at which it was consumedmaintaining about 300 psig pressure in the reactor. The reaction wascontinued in this manner until about 215 g of PSEPVE were fed to thereactor. The feeding of PSEPVE was then discontinued and VF2 was fed tothe reactor at about the rate at which it was consumed maintaining about300 psig pressure in the reactor, until an overall total of 332 g VF2had been fed to the reactor. The reactor contents were cooled to ambienttemperature, vented to 0 psig and discharged as a milky-white polymeremulsion containing 22 wt % polymer. The polymer emulsion was frozen tocause agglomeration of the polymer particles and their separation fromthe aqueous phase. The polymer agglomerates were filtered and washedvigorously 4 times with filtered tap water at about 50° C. then withdemineralized water at ambient temperature. The washed polymeragglomerates were dried at 100° C. under partial vacuum with a sweep ofnitrogen to yield 524 g of white granular polymer. DSC analysis showed aglass transition temperature at −23° C. at inflection and a well definedcrystalline melting point at 165° C. (ΔH_(f)=20.0 J/g) on the secondheat. Elemental analysis found: C, 30.33 wt % from which an averagecomposition of 8.2 mole % PSEPVE and 91.8 mole % VF2 could becalculated. The polymer could be pressed into translucent white slabsand films at 200° C. that were clean and dense, free of voids or visiblecolor.

A 100 ml flask was equipped with reflux condenser, magnetic stir bar andnitrogen inlet. The flask was charged with 5.0 g (˜5 mmole —SO₂F)PSEPVE/VF2 containing 8.4 mole % PSEPVE by ¹⁹Fnmr, 4.25 g (20 mmole)tetramethylammonium carbonate and 50 ml MeOH/H₂O (50 v %). With gentlestirring, the reaction mixture was heated to 50° C. for 24 hr, thencooled to room temperature. The polymer was collected by filtration thenwashed 4 times by suspending in distilled water at 25° C. and filtering.The collected polymer was dried at 25° C. under vacuum to yield 4.12 g(17.6% wt loss) of a discolored, dark amber powder. By ¹⁹Fnmr analysis(DMF-d7), 73% of the —SO₂F was converted and the polymer contained only5.0 mole % PSEPVE.

Comparative Example H

Hydrolysis of PSEPVE/VF2 Copolymer Using Excess TetraethylammoniumCarbonate in MeOH/H₂O

This example demonstrates that tetraethylammonium carbonate isineffective at complete hydrolysis of the sulfonyl fluoride moiety underthe conditions of Example 1.

A 100 ml flask was equipped with reflux condenser, magnetic stir bar andnitrogen inlet. The flask was charged with 5.0 g (˜5 mmole —SO₂F) of thePSEPVE/VF2 copolymer powder prepared in Comparative Example G, 6.4 g (20mmole) tetraethylammonium carbonate and 50 ml MeOH/H₂O (50 v %). Withgentle stirring, the reaction mixture was heated to 50° C. for 24 hr,then cooled to room temperature. The polymer was collected by filtrationthen washed 4 times by suspending in distilled water at 25° C. andfiltering. The collected polymer was dried at 25° C. under vacuum toyield 4.78 g (4.8% wt loss) of a white powder. By ¹⁹Fnmr analysis(DMF-d7), only 28% of the —SO₂F was converted and the polymer contained8.6 mole % PSEPVE.

1. An ionomer comprising monomer units of vinylidene fluoride and 0.5–50mole % of a perfluoroalkenyl monomer having a pendant group of theformula—(O—CF₂CFR)_(a)O—CF₂(CFR′)_(b)SO₃ ⁻NH₄ ⁺ wherein R and R′ areindependently selected from F, Cl or a perfluorinated alkyl group having1 to 10 carbon atoms, a=0, 1 or 2, b=0 to
 6. 2. The ionomer of claim 1in the form of a flat film or sheet.
 3. The ionomer of claim 1comprising 0.5–36 mole % of said perfluoroalkenyl monomer having apendant group.
 4. The ionomer of claim 1 wherein a=1, b=1, R istrifluoromethyl, and R′ is fluorine.